1. Field of the Invention
The present invention relates to an endless belt for an image forming apparatus and the image forming apparatus having the endless belt. Particularly, the present invention relates to an intermediate transfer belt used for an image forming apparatus such as a copying machine, laser beam printer or facsimile, endless belt such as a transfer material carrying belt and image forming apparatus having the endless belt.
2. Description of the Related Art
First, a case of using an endless belt for an image forming apparatus as a transfer material carrying belt is described below.
FIG. 1 is an illustration showing a schematic configuration of an image forming apparatus (color image forming apparatus) having a transfer material carrying belt.
The color image forming apparatus having the configuration shown in FIG. 1 is provided with four drum-shaped photosensitive members (photosensitive drums) (photosensitive drums 1a, 1b, 1c and 1d) serving as image bearing members.
Moreover, charging means (charging units 2a, 2b, 2c and 2d) for uniformly charging the surfaces of the photosensitive drums, exposing means (laser scanners 3a, 3b, 3c and 3d) for irradiating the surfaces of the photosensitive members with laser beams the photosensitive drums in accordance with image information and forming an electrostatic latent image on surfaces of the photoconductor drums, developing means (development units 4a, 4b, 4c and 4d) for attaching toner to the electrostatic latent image and visualizing the image as a toner image, transfer means (transfer rollers 5a, 5b, 5c and 5d) for transferring the toner image to a transfer material (paper or OHP film) S and cleaning means (cleaners 6a, 6b, 6c and 6d) for removing transfer-residual toner left on surfaces of the photosensitive drums after transfer are arranged around the photosensitive drums 1a, 1b, 1c and 1d, in that order in the rotational direction (counterclockwise rotation in FIG. 1) of the photoconductor drums to constitute image forming portions of respective colors.
Furthermore, the photosensitive drums 1a, 1b, 1c and 1d, charging units 2a, 2b, 2c and 2d, development units 4a, 4b, 4c and 4d, and cleaners 6a, 6b, 6c and 6d are integrally formed into cartridges as shown in FIG. 1 to constitute process cartridges (process cartridges 7a, 7b, 7c and 7d).
The transfer material S is carried sequentially to the image forming portions of respective colors by a transfer material carrying belt 9, toner images of respective colors are transferred and sequentially superimposed, and a synthetic toner image is formed. Then, the synthetic toner image is fixed by fixing means (fixing unit 10) and discharged to the outside of the apparatus.
Next, a case of using an endless belt for an image forming apparatus as an intermediate transfer belt is described below.
FIG. 2 is an illustration showing a schematic configuration of an image forming apparatus (color image forming apparatus) having an intermediate transfer belt.
A color image forming apparatus having the configuration shown in FIG. 2 is provided with four drum-shaped photosensitive members (photosensitive drums) (photosensitive drums 1a, 1b, 1c and 1d) serving as image bearing members.
Moreover, charging means (charging units 2a, 2b, 2c and 2d) for uniformly charging the surfaces of the photosensitive drums, exposing means (laser scanners 3a, 3b, 3c and 3d) for irradiating the surfaces of the photosensitive drums with laser beams in accordance with image information and forming an electrostatic latent image on the surfaces of the photosensitive drums, developing means (development units 4a, 4b, 4c and 4d) for attaching toner to the electrostatic latent image and visualizing the image as a toner image, primary transferring means (primary transfer rollers 5pa, 5pb, 5pc and 5pd) for primarily transferring the toner image to an intermediate transfer belt 11 and cleaning means (cleaners 6a, 6b, 6c and 6d) for removing transfer-residual toner left on surfaces of the photosensitive drums after primary transfer are arranged around the photosensitive drums 1a, 1b, 1c and 1d in that order in accordance with the rotational direction (counterclockwise rotation in FIG. 2) of the photoconductor drums to constitute image forming portions of various colors.
Furthermore, the photoconductor drums 1a, 1b, 1c and 1d, charging units 2a, 2b, 2c and 2d, development units 4a, 4b, 4c and 4d, and cleaners 6a, 6b, 6c and 6d are integrally formed into cartridges as shown in FIG. 1 to constitute process cartridges (process cartridges 7a, 7b, 7c and 7d).
Toner images of respective colors are transferred and sequentially superimposed on the intermediate transfer belt 11 and a synthetic toner image is formed in the image forming portions of respective colors.
The transfer material S is carried to a portion between the intermediate transfer belt 11 and secondary transfer means (secondary transfer roller 5s) and a synthetic toner image is transferred and fixed by fixing means (fixing unit 10) and discharged to the outside of the apparatus.
As described above, the color image forming apparatus forms a color image (full color image) by superimposing images of four colors in total from first color to fourth color (generally, yellow, magenta, cyan and black) on a transfer material or intermediate transfer belt. Therefore, when writing start positions of four colors in the sub-scanning direction (moving direction of the transfer material or intermediate transfer belt) do not coincide with each other, a problem in images referred to as a color shift occurs. Moreover, as other factors for creating a color shift, there are a shift of writing start position in the main scanning direction (a direction vertical to the moving direction of a transfer material or an intermediate transfer belt) and a color shift due to the fluctuation of the primary scanning line width.
In order to rectifying the color shift in the sub-scanning direction or main scanning direction, it has been known to form a pattern for detecting a color shift on the transfer material carrying belt or intermediate transfer belt for each color, detect the pattern by a pair of optical sensors set on both sides of the downstream portion of the transfer material carrying belt or the downstream portion of the intermediate transfer belt and perform various adjustments such as exposure timing adjustment of an image forming portion and speed adjustment of a plurality of photoconductor drums, transfer material carrying belt or intermediate transfer belt.
FIG. 3 is an illustration for explaining color shift detecting means. Though the case of a transfer material carrying belt is used as an example, the same may be applied to the case of an intermediate transfer belt.
In FIG. 3, reference numeral 20 denotes a color shift detecting means which is constituted of a light emitting element and a light receiving element. Reference numeral 21 denotes a light emitting element which is an LED for emitting, for example, light with a wavelength of an infrared region. Reference numeral 22 denotes a light receiving element (such as a photosensor). Reference numeral 9 denotes a transfer material carrying belt and 23 denotes a pattern for detecting a color shift. Reference numeral 24 denotes a light emitting optical path extended from the light emitting element 21, and 25 denotes a light receiving optical path through which reflected light from the transfer material carrying belt 9 or color shift detecting pattern 23 is received by the light receiving element 22. A light emitting portion and light receiving portion are constituted of a regular reflection optical system using the transfer material carrying belt 9 as a reflection plane to detect the position of a color shift detecting pattern in accordance with the difference between reflectances of regularly reflected lights of the transfer material carrying belt 9 and color shift detecting pattern 23, i.e., the difference between regular reflectances.
Therefore, in the case of an endless belt used for an image forming apparatus, it is necessary that the regular reflectance of the surface is high in order to increase a difference in regular reflectance with reference to toner, and moreover it is necessary that abrasion resistance is high in order to minimize a lowering in the regular reflectance of the surface due to long time use.
From such a point of view, as an endless belt for an image forming apparatus, an endless belt made of polyimide and endless belt of a multilayer configuration using a hard coat material for the outermost layer has been proposed (see Japanese Patent Application Laid-Open Nos. H11-161036 and H11-024428).
However, when using polyimide for an endless belt, cost is liable to be raised because the reaction process is long. Moreover, the surface hardness is as low as the maximum pencil hardness of 2H. Therefore, when using the endless belt for a long time, the surface is shaved by a photosensitive member, roller and transfer material, irregularity occurs and the regular reflectance of the surface of the endless belt is lowered. When the regular reflectance of the endless belt is lowered, a difference in regular reflectance with reference to toner is reduced and thereby sufficient color-shift detecting performance cannot be obtained.
Moreover, some conventional endless belts of a multilayer configuration using a hard coat material for the outermost layer use fluorocarbon resin for the outermost layer in order to improve the releasability of toner. However, because the fluorocarbon resin is a low-refraction-index material, when detecting a color shift by using regular reflection on the surface of the endless belt, the difference between regular reflectances of toner and the surface is reduced. Therefore, an error factor such as noise is increased and it is difficult to exhibit stable color-shift detecting performance.
Furthermore, when using a conventional endless belt of a multilayer configuration, the color shift detecting performance may be particularly lowered. This is because the incident light of a color shift detecting sensor passes through the outermost layer of the endless belt of a multilayer configuration and reflects on the interface between the outermost layer and the inner layer, and the reflected light on the outer surface of the outermost layer interferes with the reflected light from the interface between the outermost layer and the inner layer. Particularly, when the thickness of the outermost layer is fluctuated around integral multiples of ¼ of the wavelength of the incident light of the color shift detecting sensor, the incident light entering the photosensor of the color shift detecting sensor is greatly fluctuated and the color shift detecting performance is lowered.